Stress-free silicon wafer and a die or chip made therefrom

ABSTRACT

A stress-free wafer comprising a substrate formed of a semiconductor material having front side and back side planar and parallel surfaces and having a thickness ranging from 2 to 7 mils. The front side has electronic circuitry therein with exposed contact pads. The back side is ground and polished so that the wafer is substantially stress free and can withstand bending over a 2&#34; radius without breaking or damaging.

This invention is a continuation-in-part of application Ser. No.08/715,013 filed Sep. 17, 1996, now Pat. No. 5,733,814, which is adivision of application Ser. No. 08/415,185 filed Apr. 3, 1995, now Pat.No. 5,703,755.

This invention relates to a stress-free silicon wafer and a die madetherefrom and a method of manufacture.

Electronic cards often called Smartcards have heretofore been provided.Because of the increasing electronic requirements for such cards it hasbeen necessary to use larger and larger semiconductor dice to providethe necessary electronic functions for the card. Typically thesemiconductor die or chip utilized in such a card is formed of a rigidmaterial as for example of silicon which resists bending and has atendency to fail by cracking and/or breaking when the card is bent bythe user. In addition the silicon wafers from which such semiconductorchips or dice are made are prone to breakage during handling. Also chipsor dice cut from such wafers have ragged or rough edges and have atendency to fail after they have been placed in the field. There istherefore need for new and improved stress-free silicon wafers and chipsor dice made therefrom that can withstand severe punishment and will notbreak or fracture.

In general, it is an object of the present invention to provide astress-free silicon wafer and chips or dice made therefrom which canwithstand rigorous use without breaking and a method of manufacture forthe same.

Another object of the invention is to provide a wafer, die and method ofthe above character in which the wafer can withstand bending over aradius of 2" or less without damage to or breaking of the wafer.

Another object of the invention is to provide a wafer, die and method ofthe above character in which semiconductor dice can be readily andeconomically manufactured.

Additional objects and features of the invention will appear from thefollowing description in which the preferred embodiments are set forthin detail in conjunction with the accompanying drawings.

FIG. 1 is a plan view of the front side of the flexible electronic cardincorporating the present invention.

FIG. 2 is a plan view of the back side of the electronic card shown inFIG. 1.

FIG. 3 is a cross sectional view taken along the line 3--3 of FIG. 1.

FIG. 4 is a cross-sectional view similar to FIG. 3 but showing the useof a silicon semiconductor wafer which is embedded within the card.

FIG. 5 is a perspective view showing the manner in which a stress-freesilicon semiconductor wafer manufactured in accordance with the methodof the present invention can be bent over a radius of 2" or less withoutbreaking or fracturing.

FIG. 6 is a radial stress map of a wafer which has been ground inaccordance with the method of the present invention.

FIG. 7 is a graph showing the shape of a wafer utilized in the presentinvention before and after grinding.

In general, the stress-free silicon wafer of the present invention iscomprised of a flexible substrate formed of silicon characterized inthat it can withstand bending over a 2" radius or less without breaking.

More specifically, the present invention of a stress-free silicon waferand dice made therefrom is described in conjunction with an electroniccard 11. As shown in the drawings, the flexible electronic card 11consists of a flexible substrate formed of a suitable plastic as forexample polyethylene. It typically is sized so that it can fit within aconventional billfold. Thus it typically has a size having a length of3-3/8" and a width of 2-1/8" and a thickness of 30 to 32 mils. Inaccordance with the present invention, this thickness can range from 10to 40 mils. The plastic for making the card can be opaque or colored ifdesired. It is typically provided with many different types of indicia,some of which are visible to the human eye and some of which areinvisible to the unaided human eye.

The substrate is provided with front and back sides 13 and 14 which areplanar and parallel to each other. On the front side 13, there isprovided a rectangular space 16 which can be utilized for placing thename of the card issuer. A card number 17 is carried by the card 11which is visible to the user and typically is a number assigned to theuser and can be imprinted on the card or embossed onto the plastic ofthe card. The card also can carry a rectangular space 18 in whichcertain information can be carried, as for example the name of the userembossed in the card as well as valid dates for the card and otherdesired information. Another rectangular space 19 is provided on thecard which can carry additional identification, as for example the logoof the issuer as well as a holographic image to inhibit counterfeitingof the card. On the back side 14 of the card 11, a magnetic stripe 21extends across the card and incorporates therein certain magneticallyencoded information not visible to the user. It also includes arectangular space below the magnetic stripe 21 which carries a strip 23of the type which can be signed by the user of the card to provide anauthorized signature for checking use of the card. The remaining spaceon the back of the card can be utilized for carrying other printedinformation which the issuer may wish to place on the card.

A semiconductor device, chip, or die 26 made in accordance with thepresent invention is carried by the flexible substrate 12 and ashereinafter explained can be completely embedded in the card or can havea portion thereof exposed through either the front or back surface ofthe card. The semiconductor device 26 carries means by which it cancommunicate with an electronic card reader of a conventional type whichcan make electrical contact with the card by physically making contactwith a plurality of contacts 27 (see FIGS. 1 and 3) which are carried bythe semiconductor device 26 and which are accessible through an opening28 provided on the front side 13 of the card 11. Alternatively as shownin FIG. 4, contact can be made by an electronic card reader to thesemiconductor device through other communication means as for example aradio frequency antenna in the form of a coil 31 carried by thesemiconductor device 26 and embedded within the plastic forming thesubstrate 12.

In connection with the present invention, the semiconductor die or chiputilized is of relatively large size to provide the desired electroniccapabilities for the device. Typically it will have a minimum dimensionranging from 100 mils by 100 mils to as large as 1,000 mills by 1,000mils.

In connection with the method of the present invention, thesemiconductor die or chip 26 is fabricated from a silicon semiconductorwafer 41 as for example having a diameter ranging from 4" to 8". Beforeutilization of the method of the present invention, the semiconductorcircuits to be utilized in the semiconductor device have been formed inthe front side of the wafer by conventional diffusion and evaporationprocesses well known to those skilled in the art and during whichcontact pads, as for example the contact pads 27 hereinbefore described,have been provided on the front surface of the wafer.

Let it be assumed that the circuitry making up the semiconductor devicesformed in the wafer have been completed and that the wafer at this stageof processing has a thickness of 26 mils before back side grinding. Letit now be assumed that it is desired to thin the wafers by grinding theback side before dicing the wafers to form the desired semiconductordice or chips 26 for the electronic card 11. Such thinner wafers areeasier to dice and have improved thermal dissipation. Typically in thepast such a grinding process induced stress in the wafer and caused itto warp. Such warped wafers are more likely to break during dicing.Warped dice are more difficult to mount and are prone to break andshatter particularly when they are flexed.

In accordance with the present invention in order to providesubstantially stress-free wafers after back side grinding, the wafersare mounted face down on circular metal plates used in connection withthe grinder. These metal plates can have a suitable diameter, as forexample 10-1/2" and have a very planar surface. The wafer is mounted ona spinner. A lint-free laboratory paper (not shown) is then placed onthe planar surface of the mounting plate. The lint-free laboratory paperhas a suitable thickness, as for example 3 mils. Wax in the form of asynthetic resin dissolved in a liquid is poured into the center of thepaper. The spinner is then actuated to achieve a uniform distribution ofthe wax over the paper. Any excess is purged. The wax cover plate isthen heated until the liquid suspension solvent utilized in the wax hasbeen evaporated and all bubbles are eliminated, as for example heatingthe plate to a temperature of 150° C. After the metal plate has beenheated, the wafer with the circuit or front side down is placed over thewaxed paper. The metal mounting plate is then placed in a cold platepress. The wafers are engaged by a rubber backing plate to which arelatively high pressure is applied, as for example 2500 lbs. ofpressure on the 10-1/2" plate which by way of example can carry four 4"wafers. This ensures that the wafers are firmly pressed against themounting plate which has a very flat parallel surface. The mountingplate during this pressing operation can be subjected to cooling toensure that the wax has solidified and holds the wafers firmly in place.

The mounted plates are then placed in a conventional grinder, as forexample a Blanchard grinder. Appropriate calculations are made todetermine the depth of the grind, taking care to include the thicknessof the lint-free paper and the wax embedded therein, as for example athickness of 3 mils to allow approximately 1/2 mil for polishing. TheBlanchard grinder is then automatically set to remove the desiredmaterial from the back side, as for example descending from 26 to 8 milsin thickness.

After the grinding operation has been completed, the mounting plateswith the wafers remaining mounted thereon are subjected to a polishingprocedure by the use of a conventional polisher, as for example oneapplied by Strausbaugh and a polishing slurry. As the polishingoperation continues, the thicknesses of the wafers are periodicallymeasured until the desired thickness has been reached. After the desiredthickness has been reached, the mounting plate with the wafers thereoncan be removed from the polisher. In connection with the present method,it should be appreciated that both the grinding and polishing operationshave been carried out on the wafer without removing the wafer or wafersfrom the mounting plate.

After the polishing has been completed, the mounting plates with thewafers thereon are placed on a hot plate until the temperature of thehot plate exceeds 150° C., at which time the wafers can be pried loosefrom the mounting plates utilizing tweezers. The wafers are placed in aTeflon boat. The wafers in the boat are introduced into a vapor zone ofa vapor degreaser for a suitable period of time as for example oneminute. The wafers are then rinsed in the liquid phase of the vapordegreaser after which they are again subjected to another vapor zonetreatment and another liquid phase in the vapor degreaser. The wafersare thereafter drained and cooled. The wafers are then placed on avacuum chuck of a spinner. A cleaning solution is then introduced ontothe exposed surface of the wafer. While the wafer is spinning, a spongeis utilized to scrub the wafer after which the wafer is rinsed with DIwater and dried.

From the description of the method hereinbefore described, it can beseen that there is provided a "hot wax" method for securing the wafersduring the grinding and polishing process. No photoresist or passivationlayers are required to protect the surface of the wafers since thewafers are only mounted once on the mounting plate with the hot waxmethod. Thus, there is less wafer handling. As hereinafter explained,the method of the present invention makes it possible to produce waferswith back side grinding that are very thin and which have less stress bya factor of 10 than with conventional grinding and polishing methods.The method is also advantageous in that it tolerates the use of goldbumps and high ink dot materials on the semiconductor devices. Thepresent method can be utilized for grinding wafers from 8" (50-200millimeters) diameter to thicknesses down to 5 mils (1.25 millimeters)and even as low as 3 mils with a tolerance of plus or minus one-half mil(0.0125 millimeters).

After the grinding and polishing operations hereinbefore described havebeen completed, the wafers can be diced in a conventional manner toprovide an individual die which serves as a semiconductor device in thepresent invention. The semiconductor wafers that have been back groundin accordance with the present invention and before they are die cut canbe bent over a radius of 2" or less without fracturing or breaking asillustrated by the perspective view shown in FIG. 5. This is animportant characteristic of the wafers manufactured in accordance withthe present invention because it ensures that the dice made therefromare also flexible and will not fracture or break when embedded in theflexible electronic card of the present invention.

A radial stress map of a wafer made in accordance with the presentinvention is shown in FIG. 6 and shows the stresses induced on a waferhaving a diameter of 158 millimeters with the analysis being done on aTencor FLX laser cantilever beam system at a viewing angle of 90°. Theunits on the map as shown in FIG. 6 are in MPa×μm(stress-times-thickness). The actual stress layer thickness is 10 μm andthe stress value in MPa is equal to the values in the map divided by 10.The wafer 41 shown in the drawing had thicknesses of 18 mils and adiameter of 158 millimeters. The wafer had the following statistics:

Film Thickness -10,000 A°

Average: +1.86

Minimum +27.32

Maximum -25.71

Standard Deviation: 3.84

Viewing Angle: 90°

From the order of stress for the wafer shown in FIG. 6 which has beenback-side ground in the manner hereinbefore described, had a scale forstress which is at least one order of magnitude lower than forconventional wafers which have been back-side ground. In addition, therelative smoothness of the sawed edges, which are free of taper, die cutfrom the polished wafers which have been back side ground and polishedin the manner hereinbefore described is less than 2 microns peak-to-peakversus 4+ microns peak-to-peak for prior art methods. The radial stressmap shown in FIG. 6 shows that at least 80% of the dice cut from thewafer will be relatively stress-free in comparison to approximately 30%for wafers which have their back grinding accomplished by conventionalback grinding methods. Photographs showing a comparison of sawn edges at200 times magnification for a die made in accordance with the prior artand a die made in accordance with the present invention are shownrespectively in FIGS. 8 and 9.

In FIG. 7 there is a graph showing deflection of a wafer ground inaccordance with the present invention from 26.5 mils to 18 mils. Thedeflection is measured from a three-point support for the wafer at itsouter periphery. The solid line 51 represents deflection of the wafer at26.5 mils in thickness before grinding due to its self weight withdeflection being approximately 5 μm whereas the solid line 52 representsdeflection of the wafer due to its self weight after grinding thethickness down to 18 mils with the deflection being approximately 12micrometers. In ascertaining these deflections, the deformation of thesubstrate was ascertained by a film deposited thereon to measure thefilm stress. As is well known to those skilled in the art, stresses canbe calculated by equating the forces and moments of the film and thesubstrate.

From the foregoing, it can be seen that a method has been provided inthe present invention which results in much higher die yields per waferand therefore makes it possible to produce semiconductor dice at lowerprices to result in dramatic cost savings. In addition, the dice made inaccordance with the invention are relatively stress-free and thereforecan be bent through a substantial angle as for example an anglecorresponding to a 2", radius without fracturing or breaking. Utilizingthe same principles, it is possible to manufacture dice having athickness from 2 to 7 mils and certainly within 4 to 5 mils so that theycan be readily encapsulated in flexible plastic substrate of the typehereinbefore described to provide the flexible electronic card of thepresent invention. The semiconductor dice utilized can have largestorage capabilities and large computation capabilities. Since thewafers made in accordance with the present invention are relativelystress-free, it dices very well and the resultant dice are tough enoughto withstand extreme punishment when embedded in plastic substratesutilized for flexible electronic cards. Thus for example, suchelectronic cards can be carried in a billfold and can withstand repeatedbending which can occur in a billfold carried in the back pocket of thepants of a wearer.

What is claimed is:
 1. A stress-free wafer comprising a substrate formedof a semiconductor material having front side and back side planar andparallel surfaces and having a thickness ranging from 2 to 7 mils, saidfront side having electronic circuitry therein with exposed contactpads, said back side being ground and polished so that the wafer issubstantially stress free and can withstand bending over a 2" radiuswithout breaking or damaging.
 2. A wafer as in claim 1 wherein saidsemiconductor material has a thickness from 4 to 5 mils.
 3. A wafer asin claim 1 wherein said semiconductor material is silicon.
 4. A diecomprising a substrate formed of a semiconductor material having frontside and back side planar and parallel surfaces and having a thicknessranging from 2 to 7 mils, said front side having electronic circuitrytherein with exposed contact pads, said back side being ground andpolished so that the die is substantially stress free to inhibitcracking and breaking during use, said substrate having edges which arefree of taper and which have peak-to-peak variations of less than 2microns.
 5. A die as in claim 4 wherein said substrate has a thicknessranging from 4 to 5 mils.
 6. A die as in claim 4 wherein saidsemiconductor material is silicon.